We can see the Ste has the external clock input on pin 4 of the video port and also $820A can be set to use the externally supplied clock.

Now, Ive never seen anyone use it, or even a genlock for the ST range, but if Im seeing it all correctly, in theory, you could put a xtal (like the Falcons CAC clock/BlowUp cart does) into the Ste from the monitor port and change the shifter speed.

So I have 3 questions:

1. The TT has the same (reversed) external clock bit, (not sure about a ext clk input on the VGA pins), so can it also do this?

2. STe or TT: what are the speed +/- limits (<>31Khz) that the shifter will accept? Has anyone ever tried this?

good question,in case of ST, $FF820A - external sync means only Horizontal/Vertical synchronisation.Theoretically in STE we can supply the main clock (CPU/MMU/GLUE/SHIFTER ect) with an external clock. So in case of STE I see two questions:1) does $FF820A switch (besides Horizontal/Vertical synchronisation) also the Main clock to external2) ACIA chips need exact clock (500KHz) from Glue. What happen when the main clock will change? Will keyboard/midi working correctly?

I don't know about the TT, but on the STE this was designed exclusively for Genlock purposes. You are supposed to inject a clock with about the same frequency as the nominal main clock (32 MHz). Not sure how much you would be able to overclock the system without ill effects. Also, as noted, some clocks would be problematic.

Cyprian wrote:1) does $FF820A switch (besides Horizontal/Vertical synchronisation) also the Main clock to external

No. These are two separate switches. The external main clock is enabled with a pin on the monitor connector. See the STE hardware developer addendum.

ACIA chips need exact clock (500KHz) from Glue.

If you overclock the system you would need to replace this clock. Also the 4MHz clock might be a problem. Not sure how much you could overclock the MFP. The sound clock and the FDC clock use a separate crystal now, in the ST they were derived from the main clock.

So this input is not a seperate clock that sync locks Shifter only and the rest of system stays at 32Mhz then?

Wouldnt that screw up any advantage of genocking as the FDD/Sound/DMA/ACIA/MFP timings, etc would be trashed by the changes? Or is it the case, as Ijor was saying that keeping the alternative clock signal very close to 32Mhz wouldnt make things break?

Also, as an aside, if the external sync bit is NOT set and there is a different clock on pin 4 of video port, does the shifter then stay with the system 32Mhz clockrate?

mrbombermillzy wrote:So this input is not a seperate clock that sync locks Shifter only and the rest of system stays at 32Mhz then?

It is not. It syncs the whole systems to the injected clock. Shifter, and the ST in general, is not designed for asynchronous video output. Again, at least on STE, No idea about the TT.

Wouldnt that screw up any advantage of genocking as the FDD/Sound/DMA/ACIA/MFP timings, etc would be trashed by the changes? Or is it the case, as Ijor was saying that keeping the alternative clock signal very close to 32Mhz wouldnt make things break?

As I said, sound and FDC use a separate crystal The serial port bitrate and the system 200Hz timer use yet another separate crystal. All the other systems, would be affected. Yes, it is designed for injecting a clock at similar frequency. This is not designed for accelerators.

So yes, an external clock at 32 MHz or close should be fine, at least in theory. Don't know if there is any actual external Genlock hardware that use this feature. Anybody knows?

Also, as an aside, if the external sync bit is NOT set and there is a different clock on pin 4 of video port, does the shifter then stay with the system 32Mhz clockrate?

The external clock is selected with pin 3 of the monitor. It is not selected by software as the external sync. And again, both settings are independent. One selects if HSYNC and VSYNC would be injected externally or generated internally (by GLUE btw, not by Shifter). The other selects between the internal and external clock.

Thanks for the precise answers ijor! Although Im not a hardcore HW guy, it makes sense and I better understand the external sync bit now.

Looks like too much mucking about then, for what I had envisioned.

Besides, it looks like Troed has already done something similar with his double res hacks. On that note, it seems that the eariler ST (pre-GST) videosystem is more hackable. (Troeds hack and the double shifter high colour mod comes to mind).